1. Field of the Invention
This invention relates to an improved technique for increasing gasoline octane number and total yield while enhancing operational flexibility in catalytic cracking units by the addition of very small amounts of finely divided shape selective additive promoter to cracking catalysts.
2. Description of the Prior Art
Conversion processes utilizing crystalline zeolites have been the subject of extensive investigation during recent years, as is obvious from both the patent and scientific literature. Crystalline zeolites have been found to be particularly effective for a wide variety of organic compound conversion processes including the catalytic cracking of a gas oil to produce motor fuels. Such processes have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. It is further known from the prior art to incorporate the crystalline zeolite into a matrix for catalytic cracking and such disclosure appears in one or more of the above-identified patents.
It is also known that improved results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 Angstrom units is included with a crystalline zeolite having a pore size greater than 8 Angstrom units, either with or without a matrix. A disclosure of this type is found in U.S. Pat. No. 3,769,202. Although the incorporation of a crystalline zeolite having a pore size of less than 7 Angstrom units into a catalyst composite comprising a larger pore size crystalline zeolite (pore size greater than 8 Angstrom units) has indeed been very effective with respect to the raising of octane number, nevertheless it did so at the expense of the overall yield of gasoline.
Improved results in catalytic cracking with respect to both octane number and overall yield were achieved in U.S. Pat. No. 3,758,403. In said patent, the cracking catalyst was comprised of a large pore size crystalline zeolite (pore size greater than 7 Angstrom units) in admixture with ZSM-5 type zeolite wherein the ratio of ZSM-5 type zeolite to large pore size crystalline zeolite was in the range of 1:10 to 3:1.
The use of ZSM-5 type zeolite in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of ZSM-5 type zeolite in amounts up to and including about 5 to 10 weight percent; the latter patent discloses the weight ratio of ZSM-5 type zeolite to large pore size crystalline zeolite in the range of 1:10 to 3:1.
The criticality of using only very small amounts of a finely divided ZSM-5 class zeolite to achieve improved results with respect to octane number and overall yield has heretofore not been recognized in the art. It is the basis of the present invention that the use of only very small quantities of finely divided shape selective additive will give the same beneficial results that were once thought obtainable only by adding much larger quantities of catalyst comprising a zeolite of the ZSM-5 type preferably contained in a porous matrix.
In order to reduce automobile exhaust emissions to meet federal and state pollution requirements, many automobile manufacturers have equipped the exhaust systems of their vehicles with catalytic converters. Said converters contain catalysts which are poisoned by tetraethyl lead. Since tetraethyl lead has been widely used to boost the octane number of gasoline, refiners now have to turn to alternate means to improve gasoline octane number.
Many methods of octane improvement, however, substantially reduce the yield of gasoline. With the present short supply of available crude oil and the concomitant high demand for unleaded gasoline with a sufficiently high octane number, refiners are faced with a severe dilemma. Unfortunately, these trends are likely to continue in the foreseeable future.
One method of increasing octane number is to raise the cracking reaction temperature. This method, however, is very limited, since many units are now operating at maximum temperatures due to metallurgical limitations. Raising the cracker reactor temperature also results in increased requirements for the gas plant (i.e. gas compressor and separator). Since most gas plants are now operating at maximum capacity, any increased load could not be tolerated by the present equipment.
As can well be appreciated from the foregoing, it would be extremely desirable to have a process which will provide high octane unleaded gasoline without undue sacrifice of gasoline yield. It would be even more desirable if such results could be obtained in a simplified manner allowing an increase in operational flexibility without undue use of expensive catalysts.